1. Field of the Invention
The present invention relates to an optical information recording medium of high memory capacity in which some information is recorded or erased by elevating the temperature of an active layer by irradiation with light such as a laser beam to bring about a phase change.
2. Description of the Prior Art
Since S. R. Ovsinsky et al disclose in U.S. Pat. No. 3,530,441 that thin films of, for example, Te.sub.85 Ge.sub.15, Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2, and the like undergo a reversible phase change by irradiation with a laser beam or the like, development of optical information recording media in which a chalcogenide alloy thin film is used as an active layer and recording, reproducing and erasing can be carried out, has been hastened.
Regarding the construction of such optical information recording media, UK Patent Application GB 079031A discloses an optical information recording medium of a three-layer construction comprising an active layer comprising a chalcogenide alloy thin film and a reflection layer comprising a metallic thin film having a high light reflectivity such as Au and Al, between which an intermediate layer is inserted. This aims at enhancement of sensitivity by increasing a light absorption rate utilizing the interference effect of light. Furthermore, Heyman et al disclose constructions having an active layer, both sides of which are covered with protective layers (Philip M. Heyman, Proceedings of SPIE, Optical Storage Media, 420, 162 (1983)). Some of them have a reflection layer and the others have no reflection layer. These constructions also utilize the interference of light. However, at present, an active layer comprising a chalcogenide alloy is widely used for retaining the shape of the layer when melted by irradiation with a laser beam. One object for developing this optical information recording medium is to improve the number of rewriting cycles of recording and erasing of information. Japanese Patent Kokai (Laid-Open) No. 61-92448 discloses an optical information recording medium comprising an active layer and a resin substrate, between which is provided an intermediate layer having a coefficient of linear expansion of 1.times.10.sup.-5 /k or less. This is for inhibition of occurrence of cracks in the active layer owing to linear expansion. Moreover, Japanese Patent Kokai (Laid-Open) No. 63-187435 discloses an optical information recording medium in which a thermally conductive thin film having a higher thermal conductivity than that of the active layer is provided adjacent to the active layer. This is for reducing the difference in temperature at the central part and at the edge portion of a recording track to inhibit thermal damage and to increase the number of rewriting cycles. However, the number of rewriting cycles has still been limited in the conventional optical information recording media. This limitation is due to the generation of pinholes. (See, M. Chen, et al., Applied Physics Letters, 49, 9, 502(1986)). That is, with increase in the number of rewriting cycles, pinholes of several tens of nm in diameter are uniformly generated on the recording track of the optical information recording medium and the pinholes grow with increase of the number of rewriting cycles.